1. Field of Invention
The present invention relates to a method of fabricating a metal-oxide-semiconductor (MOS) device. More particularly, the present invention relates to a method of fabricating the integration of the high-voltage devices and the low-voltage devices.
2. Description of Related Art
As the size of the MOS devices become smaller, the operation speed of the transistors becomes faster due to the reduced channel length. However, the short channel effects derived from the reduced channel length become severe. According to the equation (electric field=voltage/channel length), the electric field is proportional to the voltage and inversely proportional to the channel length. Therefore, if the voltage applied remains constant, the intensity of the electric field will increase due to the reduced channel length, which raises the energy of the electrons in the channel and further induces electrical breakdown as well as punch-through problems.
In general, for preventing short channel effects of the high-voltage MOS device, isolation structures are mainly used, which increase the distance between the gate and the source/drain and further decrease the transverse electric field in the channel. Consequently, the MOS device can still perform normally, even during high-voltage operation.
For the integrated circuits, if the high-voltage device and the low-voltage device are to be produced simultaneously, the production process applied for the low-voltage device is limited to the level of 0.5 to 0.6 micron. Integrated circuits containing both high-voltage and low-voltage devices are not rare. For example, the low-voltage device is used in the control circuits as the high-voltage device is used in electrically programmable read only memory (EPROM), flash memory or the driving circuits of the liquid crystal display.
In order to increase the integrity of the device, it is necessary to apply a production process of a smaller size, for instance, below 0.35 micron. However, anti-punch-through doping or multi-step well doping must be applied to produce a low-voltage device in such a small size to adjust the electrical properties of the device, and to prevent the punch-through problems that occur in the low-voltage device.
Usually, the high-voltage device and the low-voltage device are manufactured separately due to different electrical properties required for the high-voltage device and the low-voltage device and then are incorporated onto the same chip. For that reason, the manufacture steps become trivial and the control over the conditions of the process becomes complicated, increasing difficulties in the control over the manufacture conditions.